This invention relates generally to centrifugal fans, and more specifically it relates to a centrifugal fan of the type which has an impeller wheel driven by a motor, the axis of the impeller being supported for rotation in a casing. The blades or passages of the impeller are arranged for centrifugally expelling a fluid towards an outlet thus forming a high pressure at a speed corresponding to the intended application of the fan.
Conventionally, a diffusing chamber is formed between the exit edges of the impeller and the inner surface of the casing. The casing has a tangentially directed passage expanding toward its outlet opening whereby the shape of the diffusing chamber depends on the efficiency of the impeller when the discharged fluid approaches the outlet opening. In the proximity of the outlet opening, the expansion of the chamber is designed such as to provide optimum velocity and pressure of the discharge fluid so that the efficiency of the fan is improved and has its maximum effect.
The mechanical performance of the fan of this type is modified according to predetermined characteristics representing different modes of operation of the fan with respect to the given operational conditions such as the desired pressure and efficiency of the fan. Some characteristics are common to all centrifugal fans, namely the characteristic curve indicating the variation of working parameters of the fan with respect to different operational conditions.
In the known types of centrifugal fans, if the rotary speed of impeller is increased, for example in order to improve the efficiency and the static pressure, the mechanical efficiency remains substantially constant since even when the increase of the rotary speed of the impeller results in an improved performance, there is also an increase in the static pressure and in the absorbed power. Accordingly, the characteristic curves show that the efficiency remains substantially constant within very narrow limits. From the laws of hydrodynamics it can be deduced that this behavior of centrifugal fans results from the fact that once a diffusion chamber has been designed for a fixed operational condition, the geometry, characteristic layout and dimensions of the chamber do not allow changes because of the fixed construction of the housing.